ocfs2: Pass ocfs2_caching_info to ocfs2_read_extent_block().

extent blocks belong to btrees on more than just inodes, so we want to
pass the ocfs2_caching_info structure directly to
ocfs2_read_extent_block().  A number of places in alloc.c can now drop
struct inode from their argument list.

Signed-off-by: Joel Becker <joel.becker@oracle.com>